In order to facilitate searching for oil deposits, determining their quality, and working them optimally, it is highly advantageous to be able to date the various levels of geological beds in which prospecting and working of deposits are undertaken.
Various methods are known for performing such dating, in particular methods that consist in measuring values of remanent magnetization in the various levels of the geological beds being prospected.
Before going any further in the present description, it is necessary to recall that the earth has a magnetic field which is presently directed towards the point called "north", but that over past geological eras, the direction of the magnetic field has switched numerous times between "north" and "south". The magnetization of terrestrial rocks subjected to the earth's magnetic field includes two components: one represents the magnetization induced by the present magnetic field of the earth, referred to by the person skilled in the art as "induced magnetization"; and the other represents "remanent" magnetization corresponding to the magnetization that was induced by the earth's magnetic field as it existed at the time the rocks were formed, such that the rocks retain a record of the field direction at that time.
Thus, to date the various levels of a geological bed, the procedure generally begins by drilling a borehole, advantageously perpendicular to the ground, and then using an apparatus that is known per se, e.g. the apparatus described in French patent FR-A-2 652 911, to measure the remanent magnetization all along the borehole, so as to determine the depths of points at which the direction of the magnetization reverses.
Since it is known when the direction of the earth's magnetic field has reversed in the past, it is theoretically easy to date each level of a bed between two points corresponding to two successive reversals of the earth's magnetic field over a period of time.
The method outlined briefly above gives good results providing the geological beds under investigation were formed uniformly merely by deposition and sedimentation, without the occurrence of phenomena and/or catastrophes such as tilting, faulting, discontinuities due to erosion, etc. Under such circumstances, the strata of geological beds are so thoroughly disturbed that interpreting the results of remanent magnetization measurements along a borehole can lead to uncertainties, gaps, and at worst, to errors in dating.